In Name Only
Anime
Adaptations that share nothing with their source material but the title, some characters' names, and maybe even the premise, to the point you could change the title and no one would tell it's an adaptation. Remakes/reboots/sequels/localizations also included.

ONA, 2018,
10 eps
Me:-
Author:1
Has the basic outline and character names of the Devilman manga, but changes everything else.
Devilman Lady
The Devil Lady

TV, 1998,
26 eps
Me:-
Author:-
Another unrelated adaptation in the Devilman franchise

TV, 2005,
39 eps
Me:-
Author:-
A few names and designs are the connection to the original Gaiking.

TV, 1996,
38 eps
Me:-
Author:-
Has nothing to do with Yuusha Raideen, even being about Sentai heroes instead of a mecha.

TV, 2007,
26 eps
Me:-
Author:-
At least this one is about mecha, but still has nothing to do with Yuusha Raideen.

TV, 2007,
12 eps
Me:-
Author:-
More of a Gravion remake than a Dancouga one.
Uchuu no Kishi Tekkaman Blade
Tekkaman Blade

TV, 1992,
49 eps
Me:-
Author:-
Shares practically nothing with the original Tekkaman.
Rokushin Gattai GodMars
God Mars

TV, 1981,
64 eps
Me:-
Author:-
Shares only character names with the Mars manga.

Movie, 2001,
1 ep
Me:-
Author:-
Combines Osamu Tezuka's Metropolis manga and Fritz Lang's Metropolis movie into a story that is neither.
Masou Kishin Cybuster
Cybuster

TV, 1999,
26 eps
Me:-
Author:-
Almost nothing to do with the Super Robot Wars games.
Project Zeorymer
Hades Project Zeorymer

OVA, 1988,
4 eps
Me:-
Author:-
Nothing to do with the Zeorymer hentai but some names.
Juubee Ninpuuchou: Ryuuhougyoku-hen
Ninja Scroll: The Series

TV, 2003,
13 eps
Me:-
Author:-
Almost nothing to do with the Juube Ninpuuchou movie.

TV, 2001,
25 eps
Me:-
Author:2
Chocobos and Cid are the one reason this is Final Fantasy.
Ged Senki
Tales from Earthsea

Movie, 2006,
1 ep
Me:-
Author:-
Minuscle similarities to the Earthsea novels.

TV, 2001,
24 eps
Me:-
Author:-
Intentionally nothing like the Galaxy Angel game due to a writer revolt.
Taiyou no Ko Esteban
The Mysterious Cities of Gold

TV, 1982,
39 eps
Me:-
Author:-
Only the names remain from The King's Fifth novel.
Sasami: Mahou Shoujo Club
Sasami Magical Girls Club

TV, 2006,
13 eps
Me:-
Author:-
Almost nothing to do with Mahou Shoujo Sammy.

TV, 1997,
12 eps
Me:-
Author:-
Bolt Crank being here is the one proof this is based on the Eat-Man manga.
Fugou Keiji: Balance:Unlimited
The Millionaire Detective – Balance: Unlimited

TV, 2020,
11 eps
Me:-
Author:-
At least it keeps the premise from the novel.
Tenchi Muyou! in Love 2: Harukanaru Omoi
Tenchi Forever!

Movie, 1999,
1 ep
Me:-
Author:-
Almsot nothing to do with the light novel that inspired it.

TV, 2008,
13 eps
Me:-
Author:-
Has a completely different plot, cast and art style from the original visual novel.

Movie, 2019,
1 ep
Me:-
Author:-
Only takes some setting elements from the Ni no Kuni games

Movie, 2019,
1 ep
Me:-
Author:-
By all means, not an adaptation of Dragon Quest V, but tries to trick you into thinking it is
Shin Megami Tensei: Tokyo Mokushiroku
Tokyo Revelation

OVA, 1995,
2 eps
Me:-
Author:3
It features SOME Megami Tensei demons I guess?

TV, 2023,
12 eps
Me:-
Author:-
Very different from both the 90s Trigun anime and the manga.
Heppoko Jikken Animation Excel♥Saga
Excel Saga

TV, 1999,
26 eps
Me:-
Author:-
Features a completely original plot not present in the manga.

Movie, 2019,
1 ep
Me:-
Author:1
Nothing to do with No Longer Human, Dazai is rolling in his grave at this being called part of his work.

TV, 2015,
12 eps
Me:-
Author:1
So divorced from the Tokyo Ghoul manga that it became infamous.
Kuroshitsuji II
Black Butler II

TV, 2010,
12 eps
Me:-
Author:-
Totally unrelated to the Kuroshitsuji manga
Rosario to Vampire Capu2
Rosario + Vampire Capu2

TV, 2008,
13 eps
Me:-
Author:-
Took the Rosario to Vampire anime in the opposite direction the manga did.
Mirai Shounen Conan 2: Taiga Daibouken
Future Boy Conan 2: River Adventure

TV, 1999,
24 eps
Me:-
Author:-
Nothing to do with the first Mirai Shounen Conan.

TV, 2006,
26 eps
Me:-
Author:-
Basically only shares basic character stuff with Tama & Friends.

Movie, 2018,
1 ep
Me:-
Author:1
Haruko is here to remind you this is supposed to be FLCL.
Kidou Senshi Gundam: Suisei no Majo
Mobile Suit Gundam: The Witch from Mercury

TV, 2022,
12 eps
Me:-
Author:-
It has basically nothing to do with the original Gundam in terms of plots, themes or tone.
Manie-Manie: Meikyuu Monogatari
Neo Tokyo

Movie, 1987,
3 eps
Me:-
Author:7
Localized as "Neo Tokyo" to make it look related to Akira given the involvement of its director
Shuranosuke Zanmaken: Shikamamon no Otoko
Sword for Truth

OVA, 1990,
1 ep
Me:-
Author:-
Originally localized as "Ninja Scroll 2" despite having no relation to Juubee Ninpuuchou
Makai Tenshou
Ninja Resurrection

OVA, 1998,
2 eps
Me:-
Author:-
Localized as "Ninja Resurrection" in another attempt to ape Juubee Ninpuuchou
Amaenaide yo!!
Ah My Buddha

TV, 2005,
12 eps
Me:-
Author:-
Localized as "Ah My Buddha" despite not being related to Aa! Megami-sama!